Thin low defect relaxed silicon germanium layers on bulk silicon substrates

ABSTRACT

A strain relaxed silicon germanium layer that has a low defect density is formed on a surface of a silicon substrate without causing wafer bowing. The strain relaxed silicon germanium layer is formed using multiple epitaxial growing, bonding and transferring steps. In the present application, a thick silicon germanium layer having a low defect density is grown on a transferred portion of a topmost silicon germanium sub-layer of an initial strain relaxed silicon germanium graded buffer layer and then bonded to a silicon substrate. A portion of the thick silicon germanium layer is then transferred to the silicon substrate. Additional steps of growing a thick silicon germanium layer having a low defect density, bonding and layer transfer may be performed as necessary.

BACKGROUND

The present application relates to a semiconductor structure and amethod of forming the same. More particularly, the present applicationrelates to a semiconductor structure containing a strain relaxed silicongermanium layer that has a low defect density, and a method of formingthe same.

Graded buffer layers (GBLs), also referred to as strain relaxed bufferlayers, are currently one of the front up approaches for 7 nm node andbeyond technologies. GBLs can enable dual channel material FinFETs on asame substrate. For example, and after the relaxed top silicon germaniumlayer (SiGe) of the GBL is formed, strained silicon can be formed on afirst portion of the relaxed top SiGe layer and in an nFET deviceregion, and a high germanium percentage SiGe alloy can be formed on asecond portion of the relaxed top SiGe layer and in a pFET deviceregion. The biggest challenge with the process and device yields is thedefect density at the surface of the GBL.

It has been shown that the thicker the SiGe layer of the GBL is, thelower the defect density at the surface of the SiGe layer is. The reasonfor that is the movement and subsequent annihilation of the threadingdislocations in the SiGe layer become easier as the thickness of theSiGe layer increases. As thicker SiGe layers are grown, a large bow ofthe silicon wafer is observed. The relaxed SiGe layer is at a largerlattice constant than the silicon substrate and as the thickness of theSiGe layer increases, the more the underlying silicon wafer gets bowed.Five micrometer to eight micrometer thick SiGe layers can be grownsafely, but for slightly thicker SiGe layers (10 micrometers to 12micrometers), there is the possibility of wafer breakage. As such, andfor a SiGe layer thickness range needed to have desired low defectdensities (less than 100 defect atoms/cm²), wafer breakage will occurdue to the large bow stress applied to the wafer. Additional acceptablebow (no wafer breakage) will create problems with semiconductor toolprocessing. Examples include wafer robot handling issues ornon-uniformity of processes due to the bowed wafer. As such, there is aneed to provide a method to form a GBL having a SiGe layer in whichdefect density is low and bowing issue is mitigated.

SUMMARY

A strain relaxed silicon germanium layer that has a low defect densityis formed on a surface of a silicon substrate without causing waferbowing. The strain relaxed silicon germanium layer is formed usingmultiple epitaxial growing, bonding and transferring steps. In thepresent application, a thick silicon germanium layer having a low defectdensity is grown on a transferred portion of a topmost silicon germaniumsub-layer of an initial strain relaxed silicon germanium graded bufferlayer and then bonded to a silicon substrate. A portion of the thicksilicon germanium layer is then transferred to the silicon substrate.Additional steps of growing a thick silicon germanium layer having a lowdefect density, bonding and layer transfer may be performed asnecessary.

In one aspect of the present application, a method of forming asemiconductor structure is provided. In one embodiment of the presentapplication, the method may include forming a silicon germanium gradedbuffer layer having a first thickness on a surface of a first siliconsubstrate, wherein the silicon germanium graded buffer layer includes aplurality of silicon germanium sub-layers in which the content ofgermanium in each of the sub-layers increases from bottom to top. Next,a topmost silicon germanium sub-layer of the silicon germanium gradedbuffer layer is bonded to a second silicon substrate, and a portion ofthe topmost silicon germanium sub-layer is transferred to the secondsilicon substrate. Next, a silicon germanium layer having a secondthickness that is greater than the first thickness is epitaxially grownon the transferred portion of the topmost silicon germanium sub-layer.Next, the silicon germanium layer having the second thickness is bondedto a third silicon substrate, and a portion of the silicon germaniumlayer having the second thickness is transferred to the third siliconsubstrate.

In another aspect of the present application, a semiconductor structureis provided. In one embodiment of the present application, thesemiconductor structure may include a strain relaxed silicon germaniumlayer having a thickness from 50 nm to 1000 nm and a defect density ofless than 100 defect atoms/cm² located directly on a surface of asilicon substrate. In the present application, the lattices of thestrain relaxed silicon germanium layer and the silicon substrate aremisaligned.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a cross sectional view of an exemplary semiconductor structureincluding a silicon germanium graded buffer layer having a firstthickness located on a surface of a first silicon substrate, wherein thesilicon germanium graded buffer layer includes a plurality of silicongermanium sub-layers in which the content of germanium in each of thesub-layers increases from bottom to top.

FIG. 2 is a cross sectional view of the exemplary semiconductorstructure of FIG. 1 after bonding a topmost silicon germanium sub-layerof the silicon germanium graded buffer layer to a second siliconsubstrate.

FIG. 3 is a cross sectional view of the exemplary semiconductorstructure of FIG. 2 after transferring a portion of the topmost silicongermanium sub-layer to the second silicon substrate.

FIG. 4 is a cross sectional view of the exemplary semiconductorstructure of FIG. 3 after epitaxially growing a silicon germanium layerhaving a second thickness that is greater than the first thickness onthe transferred portion of the topmost silicon germanium sub-layer.

FIG. 5 is a cross sectional view of the exemplary semiconductorstructure of FIG. 4 after bonding the silicon germanium layer having thesecond thickness to a third silicon substrate.

FIG. 6 is a cross sectional view of the exemplary semiconductorstructure of FIG. 5 after transferring a portion of the silicongermanium layer having the second thickness to the third siliconsubstrate.

FIG. 7 is a cross sectional view of the exemplary semiconductorstructure of FIG. 6 after epitaxially growing a silicon germanium layerhaving a third thickness on the transferred portion of the silicongermanium layer having the second thickness.

FIG. 8 is a cross sectional view of the exemplary semiconductorstructure of FIG. 7 after bonding the silicon germanium layer having thethird thickness to a fourth silicon substrate, and transferring aportion of the silicon germanium layer having the third thickness to thefourth silicon substrate.

DETAILED DESCRIPTION

The present application will now be described in greater detail byreferring to the following discussion and drawings that accompany thepresent application. It is noted that the drawings of the presentapplication are provided for illustrative purposes only and, as such,the drawings are not drawn to scale. It is also noted that like andcorresponding elements are referred to by like reference numerals.

In the following description, numerous specific details are set forth,such as particular structures, components, materials, dimensions,processing steps and techniques, in order to provide an understanding ofthe various embodiments of the present application. However, it will beappreciated by one of ordinary skill in the art that the variousembodiments of the present application may be practiced without thesespecific details. In other instances, well-known structures orprocessing steps have not been described in detail in order to avoidobscuring the present application.

It will be understood that when an element as a layer, region orsubstrate is referred to as being “on” or “over” another element, it canbe directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “beneath” or “under” another element, it can bedirectly beneath or under the other element, or intervening elements maybe present. In contrast, when an element is referred to as being“directly beneath” or “directly under” another element, there are nointervening elements present.

Referring first to FIG. 1, there is illustrated an exemplarysemiconductor structure that can be employed in accordance with anembodiment of the present application. The exemplary semiconductorstructure shown in FIG. 1 includes a silicon germanium graded bufferlayer 10 having a first thickness located on a surface of a firstsilicon substrate 8. The first thickness of the silicon germanium gradedbuffer layer 10 may be in a range from 5 micrometers to 8 micrometers.The first silicon substrate 8 is a bulk silicon substrate (i.e., theentirety of the substrate is composed of silicon). The first siliconsubstrate 8 is typically single crystalline.

The silicon germanium graded buffer layer 10 is strain relaxed and thesilicon germanium graded buffer layer 10 includes a plurality of silicongermanium sub-layers (e.g., 12, 14, 16, 18 and 20). In one embodiment,the silicon germanium graded buffer layer 10 includes 5 sub-layers asshown in FIG. 1. The number of sub-layers of the silicon germaniumgraded buffer layer 10 may vary and is not limited to five sub-layers.

The silicon germanium graded buffer layer 10 is step graded. The term“step graded” denotes that the content of germanium within the silicongermanium graded buffer layer 10 increases in a non-abrupt manner upwardfrom the interface with the silicon substrate 8. That is, the content ofgermanium within each silicon germanium sub-layer that provides thesilicon germanium graded buffer layer increases from bottom to top.

The topmost silicon germanium sub-layer (e.g., sub-layer 20) of thesilicon germanium graded buffer layer 10 has a final desired germaniumcontent. The topmost silicon germanium sub-layer (e.g., sub-layer 20) ofthe silicon germanium graded buffer layer 10 also has a greaterthickness than the other sub-layers (e.g., sub-layers 12, 14, 16, 18) ofthe silicon germanium graded buffer layer 10. In one example, thethickness of the topmost silicon germanium sub-layer (e.g., sub-layer20) of the silicon germanium graded buffer layer 10 is from 2micrometers to 4.5 micrometers.

In the illustrated embodiment, silicon germanium sub-layer 12, which maybe referred to as a bottommost sub-layer of the silicon germanium gradedbuffer layer 10, has the lowest germanium content. In one embodiment,the germanium content of the bottommost sub-layer (e.g., sub-layer 12)of the silicon germanium graded buffer layer 10 can be in a range from 2atomic percent germanium to 6 atomic percent germanium. The bottommostsub-layer (e.g., sub-layer 12) of the silicon germanium graded bufferlayer 10 may have a thickness that can range from 600 nm to 1000 nm.

The silicon germanium sub-layers (e.g., sub-layers 14, 16, 18) that arelocated between the bottommost silicon germanium sub-layer 12 and thetopmost silicon germanium sub-layer (e.g., sub-layer 20) may have athickness in the range mentioned above for the bottommost silicongermanium sub-layer 12. The thickness of silicon germanium sub-layers(e.g., sub-layers 14, 16, 18) that are located between the bottommostsilicon germanium sub-layer 12 and the topmost silicon germaniumsub-layer (e.g., sub-layer 20) may be the same as, or different from,the thickness of the bottommost silicon germanium sub-layer 12.

The silicon germanium sub-layer 14 has a higher germanium content thanthe bottommost silicon germanium sub-layer 12. In one example, thegermanium content of the silicon germanium sub-layer 14 is from 6 atomicpercent germanium to 12 atomic percent germanium. The silicon germaniumsub-layer 16 has a higher germanium content than the silicon germaniumsub-layer 14. In one example, the germanium content of the silicongermanium sub-layer 16 is from 12 atomic percent germanium to 16 atomicpercent germanium. The silicon germanium sub-layer 18 has a highergermanium content than the silicon germanium sub-layer 16. In oneexample, the germanium content of the silicon germanium sub-layer 18 isfrom 16 atomic percent germanium to 20 atomic percent germanium. Thetopmost silicon germanium sub-layer (e.g., sub-layer 20) has a highergermanium content than the silicon germanium sub-layer 18. In oneexample, the germanium content of the topmost silicon germaniumsub-layer (e.g., sub-layer 20) is from 20 atomic percent germanium to 25atomic percent germanium.

The silicon germanium graded buffer layer 10 may have a topmost layer(surface) defect density of from 5×10⁴ defect atoms/cm² to 1×10⁷ defectatoms/cm². The silicon germanium graded buffer layer 10 may containthreading dislocation defects that extend upward from the bottommostsilicon germanium sub-layer (e.g., sub-layer 12) to the topmost silicongermanium sub-layer (e.g., sub-layer 20).

The silicon germanium graded buffer layer 10 is formed on a surface ofthe first silicon substrate 8 utilizing an epitaxial growth (ordeposition) process. The terms “epitaxial growth and/or deposition” and“epitaxially formed and/or grown” mean the growth of a material on adeposition surface of a material, in which the material being grown hasthe same crystalline characteristics as the material of the depositionsurface. In an epitaxial deposition process, the chemical reactantsprovided by the source gases are controlled and the system parametersare set so that the depositing atoms arrive at the deposition surface ofthe growth surface with sufficient energy to move around on the surfaceand orient themselves to the crystal arrangement of the atoms of thedeposition surface. Therefore, an epitaxial material has the samecrystalline characteristics as the deposition surface on which it isformed. In the present application, the silicon germanium graded bufferlayer 10 including each of the sub-layers (e.g., sub-layers 12, 14, 16,18 and 20) has an epitaxial relationship, i.e., same crystallinecharacteristic, as the silicon substrate 8.

Examples of various epitaxial growth process apparatuses that aresuitable for use in forming the silicon germanium graded buffer layer 10include, e.g., rapid thermal chemical vapor deposition (RTCVD),low-energy plasma deposition (LEPD), ultra-high vacuum chemical vapordeposition (UHVCVD), atmospheric pressure chemical vapor deposition(APCVD) and molecular beam epitaxy (MBE). The temperature for epitaxialdeposition typically ranges from 550° C. to 900° C. Although highertemperature typically results in faster deposition, the fasterdeposition may result in crystal defects and film cracking.

A number of different sources may be used for the deposition of eachsub-layer that provides the silicon germanium graded buffer layer 10. Insome embodiments, the source gas for the deposition of each silicongermanium sub-layer (e.g., sub-layers 12, 14, 16, 18, 20) of the silicongermanium graded buffer layer 10 may include an admixture of a siliconcontaining gas source and a germanium containing gas source. Examples ofsilicon containing gas sources that may be employed include silane,disilane, trisilane, tetrasilane, hexachlorodisilane, tetrachlorosilane,dichlorosilane, trichlorosilane, and combinations thereof. Examples ofgermanium containing gas sources that may be employed include germane,digermane, halogermane, dichlorogermane, trichlorogermane,tetrachlorogermane and combinations thereof. In some embodiments, eachsilicon germanium sub-layer (e.g., sub-layers 12, 14, 16, 18, 20) of thesilicon germanium graded buffer layer 10 can be formed from a source gasthat includes a compound containing silicon and germanium. Carrier gaseslike hydrogen, nitrogen, helium and argon can be used.

Referring now to FIG. 2, there is illustrated the exemplarysemiconductor structure of FIG. 1 after bonding the topmost silicongermanium sub-layer (e.g., sub-layer 20) of the silicon germanium gradedbuffer layer 10 to a second silicon substrate 22. The second siliconsubstrate 22 is also a bulk silicon substrate and it is typically singlecrystalline.

Bonding may be performed utilizing any wafer-to-wafer bonding processwell known to those skill in the art. In one embodiment, bonding may beachieved by first bringing the topmost silicon germanium sub-layer(e.g., sub-layer 20) of the silicon germanium graded buffer layer 10into intimate contact with the second silicon substrate 22, andthereafter a bonding anneal may be performed. In one embodiment, thebonding anneal may be performed at a temperature from 200° C. to 600° C.In some embodiments, the bonding anneal may be omitted. An externalforce may or may not be applied during the bonding process.

In some embodiments, and prior to wafer bonding, a weakened or “bubble”layer is formed within the topmost sub-layer (e.g., sub-layer 20) of thesilicon germanium graded buffer layer 10 by ion implantation. Theweakened or “bubble” layer can be used to subsequently transfer a thinportion of the topmost sub-layer (e.g., sub-layer 20) of the silicongermanium graded buffer layer 10 to the second silicon substrate 22. Insuch an embodiment, the transfer may occur during the bonding annealprocess by initiating breakage at the weakened or “bubble” layer withinthe topmost sub-layer (e.g., sub-layer 20) of the silicon germaniumgraded buffer layer 10.

Referring now to FIG. 3, there is illustrated the exemplarysemiconductor structure of FIG. 2 after transferring a portion of thetopmost silicon germanium sub-layer (e.g., sub-layer 20) to the secondsilicon substrate 22. The transferred portion of the topmost silicongermanium sub-layer (e.g., sub-layer 20) may be referred to herein as atopmost silicon germanium sub-layer portion (e.g., sub-layer portion20′). The thickness of the topmost silicon germanium sub-layer portion(e.g., sub-layer portion 20′) may be from 50 nm to 1000 mm. The topmostsilicon germanium sub-layer portion (e.g., sub-layer portion 20′) hasthe same germanium content as the topmost silicon germanium sub-layer(e.g., sub-layer 20).

Transferring may be performed during the bonding process itself byforming the weakened or “bubble” layer within the topmost sub-layer(sub-layer 20) of the silicon germanium graded buffer layer 10 and/orafter bonding has been performed. When transferring is performed afterthe bonding, the transferring may be performed by a chemical removalprocess such as, for example, planarization (i.e., chemical mechanicalpolishing and/or grinding).

Referring now to FIG. 4, there is shown the exemplary semiconductorstructure of FIG. 3 after epitaxially growing a silicon germanium layer24 having a second thickness that is greater than the first thickness onthe transferred portion of the topmost silicon germanium sub-layer(e.g., topmost silicon germanium sub-layer portion 20′). In one example,the second thickness of the silicon germanium layer 24 is from 5micrometers to 8 micrometers. The silicon germanium layer 24 is strainrelaxed.

The germanium content of the silicon germanium layer 24 is within therange mentioned above for the topmost silicon germanium sub-layer (e.g.,sub-layer 20) of the silicon germanium graded buffer layer 10. Epitaxialgrowth of silicon germanium layer 24 includes the conditions,apparatuses and source gases mentioned above for forming each silicongermanium sub-layer (e.g., sub-layers 12, 14, 16, 18, 20) of the silicongermanium graded buffer layer 10. Silicon germanium layer 24 has a lowerdefect density and thus less threading dislocation defects as comparedwith the silicon germanium graded buffer layer 10. In some embodimentsin which silicon germanium layer 24 reaches a thickness within the abovementioned range, the threading dislocation defects within the silicongermanium layer 24 are entirely annihilated. In some embodiments, thedefect density of the silicon germanium layer 24 is less than 100 defectatoms/cm².

Referring now to FIG. 5, there is illustrated the exemplarysemiconductor structure of FIG. 4 after bonding the silicon germaniumlayer 24 having the second thickness to a third silicon substrate 26.The third silicon substrate 26 is also a bulk silicon substrate and itis typically single crystalline. Bonding may be performed as describedabove.

Referring now to FIG. 6, there is illustrated the exemplarysemiconductor structure of FIG. 5 after transferring a portion of thesilicon germanium layer 24 having the second thickness to the thirdsilicon substrate 26. The transferred portion of the silicon germaniumlayer 24 may be referred to herein as silicon germanium layer portion24′. The thickness of the silicon germanium layer portion 24′ that istransferred may be from 50 nm to 1000 mm. The silicon germanium layerportion 24′ has the same germanium content as the silicon germaniumlayer 24.

Transferring may be performed during the bonding process itself byforming the weakened or “bubble” layer within the silicon germaniumlayer 24 and/or after bonding has been performed. When transferring isperformed after the bonding, the transferring may be performed by achemical removal process such as, for example, planarization (i.e.,chemical mechanical polishing and/or grinding).

Referring now to FIG. 7, there illustrated the exemplary semiconductorstructure of FIG. 6 after epitaxially growing a silicon germanium layer28 having a third thickness on the transferred portion of the silicongermanium layer 24 (i.e., silicon germanium layer portion 24′). Thethird thickness of the silicon germanium layer 28 may be the same as, orgreater than, the second thickness mentioned above for silicon germaniumlayer 24. In one example, the third thickness of the silicon germaniumlayer 28 is from 5 micrometers to 8 micrometers. The silicon germaniumlayer 28 is strain relaxed.

The germanium content of the silicon germanium layer 28 is within therange mentioned above for the topmost silicon germanium sub-layer (e.g.,sub-layer 20) of the silicon germanium graded buffer layer 10. Epitaxialgrowth of silicon germanium layer 28 includes the conditions,apparatuses and source gases mentioned above for forming each silicongermanium sub-layer (e.g., sub-layers 12, 14, 16, 18, 20) of the silicongermanium graded buffer layer 10. Silicon germanium layer 28 has a lowerdefect density and thus less threading dislocation defects as comparedwith the silicon germanium graded buffer layer 10. In some embodimentsin which silicon germanium layer 28 reaches a thickness within the abovementioned reach, the threading dislocation defects within the silicongermanium layer 28 are entirely annihilated. In some embodiments, thedefect density of the silicon germanium layer 28 is less than 100 defectatoms/cm².

Referring now to FIG. 8, there is illustrated, the exemplarysemiconductor structure of FIG. 7 after bonding the silicon germaniumalloy 28 having the third thickness to a fourth silicon substrate 30,and transferring a portion of the silicon germanium layer 28 having thethird thickness to the fourth silicon substrate 30. The fourth siliconsubstrate 30 is also a bulk silicon substrate and it is typically singlecrystalline. Bonding and layer transferring may be performed asdescribed above. The transferred portion of the silicon germanium layer28 may be referred to herein as silicon germanium layer portion 28′. Thethickness of the silicon germanium layer portion 28′ may be from 50 nmto 1000 mm. The silicon germanium layer portion 28′ has the samegermanium content as the silicon germanium layer 24.

Notably, FIG. 7 shows one exemplary semiconductor structure of thepresent application which includes a strain relaxed silicon germaniumlayer (i.e., silicon germanium layer portion 28′) having a thicknessfrom 50 nm to 1000 nm and a defect density of less than 100 defectatoms/cm² located directly on a surface of a silicon substrate (i.e.,fourth silicon substrate 30). The lattices of the strain relaxed silicongermanium layer (i.e., silicon germanium layer portion 28′) and thesilicon substrate (i.e., fourth silicon substrate 30) are misaligned. Inthe present application, a dislocation network is present at aninterface of the strain relaxed silicon germanium layer (i.e., silicongermanium layer portion 28′) and the silicon substrate (i.e., fourthsilicon substrate 30).

Moreover, no bowing of the exemplary semiconductor structure isobserved. Also, it is noted that the various silicon substrates (e.g.,substrates 8, 22, 26) that were used and disregarded may be re-used.

Additional steps of silicon germanium growth, bonding and transferringmay be used as desired to provide a final semiconductor structure havingthe properties mentioned above. Also, it is possible to use on one cycleof silicon germanium growth, bonding and transferring. Thus, it ispossible to stop the process after forming the structure shown in FIG.6.

While the present application has been particularly shown and describedwith respect to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formsand details may be made without departing from the spirit and scope ofthe present application. It is therefore intended that the presentapplication not be limited to the exact forms and details described andillustrated, but fall within the scope of the appended claims.

1. A method of forming a semiconductor structure, said methodcomprising: forming a silicon germanium graded buffer layer having afirst thickness on a surface of a first silicon substrate, wherein saidsilicon germanium graded buffer layer includes a plurality of silicongermanium sub-layers in which the content of germanium in each of saidsub-layers increases from bottom to top; bonding a topmost silicongermanium sub-layer of said silicon germanium graded buffer layer to asecond silicon substrate; transferring a portion of said topmost silicongermanium sub-layer to said second silicon substrate; epitaxiallygrowing a silicon germanium layer having a second thickness that isgreater than said first thickness on said transferred portion of saidtopmost silicon germanium sub-layer; bonding said silicon germaniumlayer having said second thickness to a third silicon substrate; andtransferring a portion of said silicon germanium layer having saidsecond thickness to said third silicon substrate.
 2. The method of claim1, further comprising: epitaxially growing a silicon germanium layerhaving a third thickness on said transferred portion of said silicongermanium layer having said second thickness; bonding said silicongermanium layer having said third thickness to a fourth siliconsubstrate; and transferring a portion of said silicon germanium layerhaving said third thickness to said fourth silicon substrate.
 3. Themethod of claim 1, wherein said forming said silicon germanium gradedbuffer layer comprises epitaxial growth of said plurality of silicongermanium sub-layers.
 4. The method of claim 1, wherein said topmostsilicon germanium sub-layer of said silicon germanium graded bufferlayer has a germanium content from 20 atomic percent germanium to 25atomic percent germanium, and a thickness from 2 micrometers to 4.5micrometers.
 5. The method of claim 3, wherein said silicon germaniumlayer of said second thickness has a germanium content from 20 atomicpercent germanium to 25 atomic percent germanium.
 6. The method of claim1, wherein said transferred portion of said silicon germanium layerhaving said second thickness is from 50 nm to 1000 nm thick.
 7. Themethod of claim 6, wherein said transferred portion of said silicongermanium layer having said second thickness has a defect density ofless than 100 defect atoms/cm².
 8. The method of claim 6, wherein saidtransferred portion of said silicon germanium layer having said secondthickness is void of threading dislocation defects.
 9. The method ofclaim 1, wherein said transferring said portion of said topmost silicongermanium sub-layer comprises forming a weakened or bubble region insaid topmost silicon germanium sub-layer by ion implantation.
 10. Themethod of claim 1, wherein said transferring said portion of saidtopmost silicon germanium sub-layer comprises a material removalprocess.
 11. The method of claim 1, wherein said transferring saidportion of said silicon germanium layer having said second thicknesscomprises forming a weakened or bubble region in said silicon germaniumlayer having said second thickness by ion implantation.
 12. The methodof claim 1, wherein said transferring said portion of said silicongermanium layer having said second thickness comprises a materialremoval process.
 13. The method of claim 2, wherein said silicongermanium layer of said third thickness has a germanium content from 20atomic percent germanium to 25 atomic percent germanium.
 14. The methodof claim 13, wherein said transferred portion of said silicon germaniumlayer having said third thickness is from 50 nm to 1000 nm thick.15.-18. (canceled)